What causes belief? Or, rather, who causes belief?
If you think it is something a fallen man, with a heart of stone, can produce from his own resources, without a heart of flesh being given to him, then you belong in the free willer camp, and you have reason to boast.
If you think it is God who gives the person a heart of flesh, which produces, as a fruit, faith and repentance, then you belong in the Reformed camp, and know that you have zero reason to boast.
That's really the determining factor.
Ephesians 2:1-10 is plain in this regard, I think, and no man has any reason for boasting, even his .1% that free willers think he has contributed to his salvation, as the determining factor.
I'm definitely in the latter camp
Praise be to God!
By the way, you may have been addressing someone else. That would be my response, though.
Additionally, the gospel is part of the means that God has decreed. He has decreed both the end and the means. That way, the regenerate man knows that the gospel is the means by which God had decreed to save him.